Light emitting diode (LED) mainly includes multiple layers of semiconductor materials formed by epitaxy. Take a blue LED as an example, it mainly formed by III-nitride based epitaxial films.
Please refer to FIG. 1 for a conventional vertical LED. It comprise an N-type semiconductor layer 1, a light emission layer 2 and a P-type semiconductor layer 3, and under the P-type semiconductor layer 3 in this order a mirror layer 4, a buffer layer 5, a bonding layer 6, a silicon substrate 7 and a P-type electrode 8. The surface of the N-type semiconductor layer 1 can be treated in a coarse process to enhance light extraction efficiency. The LED also can have an N-type electrode 9 located thereon. When a voltage is applied to the N-type electrode 9 and P-type electrode 8, the N-type semiconductor layer 1 provides electrons, and the P-type semiconductor layer 3 provides electric holes, then the electrons and electric holes are joined in the light emission layer 2 to generate light.
The N-type electrode 9 generally is made of metal, such as silver, aluminum, nickel or the like. The N-type electrode 9 is easily softened and condensed to become spherical shape due to high temperature and results in peeling off. After used for a period of time, metal ions slowly permeate into the N-type semiconductor layer 1 that causes unstable electric property and results in shortening lifespan of the LED. It still leaves a lot to be desired, especially in terms of durability.